trigger trgOnInsertEntitlement on Entitlement__c (before insert) { 
    for(Entitlement__c e:trigger.new) { 
        if (e.Case_Pack__c==True) {
            if (e.Renewal_Amount__c==null) e.Renewal_Amount__c = 0; 
        } else {
            if (e.Renewal_Amount__c==0) e.Renewal_Amount__c = null;
        } 
    }
}